Page 53 - Oil and Gas Production Handbook An Introduction to Oil and Gas Production

The operating characteristics are defined by the manufacturer. In the above
diagram the blue lines mark constant speed lines. The maximum operating
limits are set by the orange line as described above. The surge domain is
the area to the left of the red surge curve.

The objective of compressor performance control is to keep the operating
point close to the optimal set point without violating the constraints, by
means of control outputs, such as the speed setting. However, gas turbine
speed control response is relatively slow and even electric motors are not
fast enough, since surge response must be in the 100 ms range. Anti surge
control will protect the compressor from going into surge by operating the
surge control valve. The basic strategy is to use distance between operating
point and surge line to control the valve with a slower response time starting
at the surge control line. Crossing the surge trip line will cause a fast
response opening of the surge valve to protect the compressor.

Operation with recirculation wastes energy (which could result in
unnecessary emissions) and produces wear and tear, particularly on the
surge valve. Each vendor supplies several variants of compressor control
and anti surge control to optimize performance, based on various corrective
and predictive algorithms. Some strategies include:

• Set point adjustment: If rapid variations in load cause surge valve
action, the set point will be moved to increase the surge margin.

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